The present invention relates to the improvement of an electro-magnetic wave absorber device, or the improvement of a device for preventing the leakage of waves. The present invention is used, for instance, for preventing leakage of wave energy in a microwave heater, for example, a microwave oven.
Conventionally, an absorber device for microwave leakage in a microwave oven has three absorber means. The first one is a metal contact spring which provides the conductive contact between the body and the door to close the door completely. The second one is a choke cavity with 1/4 wavelength for absorbing waves which leak through said conductive contact. The third one is a ferrite absorber provided at the outlet of the leakage path for absorbing the rest of the leakage.
The present applicant obtained the U.S. Pat. No. 4,525,614 (the corresponding UK Pat. No. 2 122 059, French Pat. No. 82 19333, German Pat. No. 32 42 125). That patent has the feature that the opening of the choke cavity is covered with ferrite material. That patent is excellent as far as the fundamental oscillation frequency (2450 MHz) of a microwave oven concerned.
However, it has been lately become desirable that not only the fundamental frequency but also the harmonic frequencies (for instance the frequency around 12 GHz) are sufficiently attenuated so that a satellite broadcast is not disturbed by a microwave oven. Further, the high frequency noise between 1 GHz and 18 GHz is subject to control.
Accordingly, the leakage of not only the fundamental frequency but also the harmonics must be prevented.
A prior choke cavity which resonates with the fundamental frequency is useless to prevent the subsequent harmonics.